Guide tubes may be used to support instruments disposed within them. Many conventional guide tubes provide some form of locking capability. However, conventional guide tubes provide a single surface lock engagement when locking. Such locking configurations may provide locking forces up to a point but are generally limited in the amount of locking force that may be generated. Some emerging forms of surgery may benefit from guide tubes or controllable instruments with locking or braking capabilities that provide not only improved articulation and control but also increased locking force.